Electrophotography has become one of the most widely utilized systems in the field of information processing. In particular, dry copying or xerography as it is otherwise known, uses electrophotography in creating copies of documents and other materials. Electrophotography has become the stardard process for making copies of documents or other materials in a host of environments including business offices, educational institutions and the like. The basic principles of electrophotography are well known to those skilled in the art.
The fundamental elements of an electrophotographic copier include a photoreceptive medium which is charged to a substantially uniform level. An optical image of an original to be copied is focused onto the photoreceptive medium through the use of a light source and appropriate optics. The optical image of the original illuminates the charged photoreceptive medium thereby dissipating the charge on the photoreceptive medium in accordance with the intensity of the light in the optical image. An electrostatic latent image corresponding to the original is left on the photoreceptive medium.
In most electrophotographic copying systems, the electrostatic latent image is passed through a development station which includes a source of toner materials electrostatically held to ferromagnetic carriers. Magnetic fields acting on the ferromagnetic carries bring the toner materials into contact with the electrostatic latent image. The electrostatic charge on the latent image is strong enough to pull the toner materials away from the ferromagnetic carriers and to hold them in place on the appropriate portions of the electrostatic latent image. The magnetic forces also carry the ferromagnetic carrier particles back to a position where they are remixed with additional toner materials.
As is known to those skilled in the art, the toner materials are normally plastics which melt at a predetermined temperature and have appropriate color characteristics once they are melted.
Having passed through the developed station, the electrostatic latent image is now referred to as the developed image. Subsequently, the photoreceptive medium carrying the developed image is brought into contact with an receptor which, in the most common application of electrophotography, is a sheet of paper. Electrostatic charging techniques are again employed in transferring the toner of the developed image from the photoreceptive medium to the image receptor.
Once the developed image is transferred to the image receptor, it is passed through a device commomly referred to as a fuser. The fuser is a station in the path of the image receptor at which the transferred toner is heated to fix the image onto the image receptor. By this process a monochromatic copy of the original image is created.
In recent years, systems for color electrophotography have been created for the purpose of making color copies of color originals. The systems for color electrophotography are generally analogous to standard three-color printing processes used in the more conventional printing arts. Three different color component electrostatic latent images are created through the illumination of the color original through three separate filters in a manner analogous to the creation of color separations in color printing. Each of the color component electrostatic latent images is developed with a toner having the appropriate color characteristics. Each developed color component image is in turn transferred to the image receptor or paper in superimposed registration with one another in order to provide a composite image. The image receptor carrying the composite image is then passed through a fuser in a conventional manner where the toner is fixed to the image receptor. A system for color electrophotography is more particularly disclosed in the commonly owned patent to Palm, U.S. Pat. No. 4,652,115 for a print engine for color electrophotography, which is incorporated herein by reference.
As discussed above, electrophotographic copying systems have been developed to reproduce color or monochromatic originals. And as discussed in Palm, supra, it is well known in the art to provide an electrophotographic print engine which can reproduce either color or monochromatic originals. However, original documents or materials also may be classified according to whether they are graphic materials or pictorial materials. Graphic materials include textual documents, graphs, and charts wherein subtle variations in shading, tone or color are not present. Contrary thereto, pictorial materials include photographs, drawings and illustrations which may possess subtle variations in shading, tone and color.
Machines desined primarily for graphic and textual reproduction employ high contrast imagery production techniques. These are also referred to as high gamma characteristics, the well known gamma characteristics of print engines being discussed in more detail herein below. In particular, at low density values for an original image, an electrophotographic engine which sharply reduces the copied inage will have bright white (or unfilled) areas and contrast with dark filled areas. Indeed, the contrast in such machines is often exaggerated with respect to the original. This produces the desired quality of good "sharpness" of the copied image.
When photographs or other pictorial source materials are reproduced on such copying machines the image is distorted and undesirable in that a wide range of image densities in the original are reproduced as a narrow range of image densities in the copy, leading to a "muddy" appearance in the pictorial copy. Additionally, high contrast modes of operation on a copying machine exhibit a flattening of the gamma curve at high image densities which may be considered analogous to the phenomenon of clipping in an electronic amplifier. Thus, variations in shading of generally dense portions of the image original will be lost and, after a certain original image density level is reached, all areas in the copy at or above that density will be reproduced at a single high density value, thus further tensing to muddy the pictorial image. Adjustments to gamma characteristics to form better pictorial reproduction lead to lower contrast graphic images which users objectively view as less sharp and less desirable than textual or graphich images reproduced by high contrast machines. The practial results of the phenomenon are familiar to user's of electrophotographic copiers and those skilled in the art.
The subtle variations in shading, tone and color have made it difficult for prior art electrophotohraphy systems to reproduce acceptable copies of pictorial materials. In order to aid electrophotographic copying systems in forming tone gradations, it has been well known in the art to employ a screen in the optical path of an electrophotographic print engine. The screen is positioned usually between the original to be imaged and the photoreceptive medium. Generally, a screen comprises a transparent base medium having a plurality of opaque dots or lines printed thereon. When the screen is in place, the portions of the optical image corresponding to the opaque areas of the screen are blocked from illuminating the photoreceptive medium.
The effect of use of the screen is to produce tonal gradations on the electrostatic latent image by forming half-tone dots or lines of varying size. In the highlight regions where the toner should be less densely applied, the electrostatic latent image formed through use of the screen may comprise narrow lines or small dots. The lines increase in width or the dots in size throughout the intermediate shades until they merge together in the darker areas where toner should be applied most densely. In this manner, there will be completely whiteness in the lightest areas with increasing denseness of applied toner through the intermediate shades and nearly solid black or solid color in the densest areas. The grid thus serves as a diffraction grating.
The density transfer characteristic of an electrophotographic copying machine is often represented in tone reproduction curves (TRCs). Standard tone reproduction curves are Cartesian graphs relating source and image density. A TRC, also known as a gamma curve, is derived from plotting the image density of the original against the image density of the copy. The input tone density is expressed in terms of log.sub.10 (100/Ro), where Ro is the percent spectral reflectivity of the original. The output tone density is expressed in terms of log.sub.10 (100/Rc) where Rc is the percent spectral reflectivity of the copy. Thus, where the reflectivity approaches 100% (white areas), the tone density approaches 0 (log.sub.10 100/100=0). Where the reflectivity decreases, (colored or shaded areas), the tone density increases.
The gamma characteristic noted above is the ratio of the output density value to the input density value on the tone reproduction curve at any point. From this, it will be apparent that an ideal copying machine would have a gamma of one for all values of input image density and its tone reproduction curve would be a straight line at 45.degree. on a graph having equal unit spacing on the horizontal and vertical axes. It is well known to those skilled in the art that such an idealized gamma curve is virtually impossible to achieve. This results from the fact that the development process is a linear function of photoreceptor voltage while the exposure process includes a nonlinear function of image density. To be more precise, the density of toner pulled at a development station is a linear function of the voltage on the photoreceptor, which voltage exerts electrostatic force on the charged toner particles in the development mechanism. The discharge of an evenly charged photoreceptor belt varies as the square root of the exposure. This is explained in detail in an article entitled "Problems of Pictorial Xerography" by R. N. Goren in the Winter 1976 issue (Vol. II, No. 1) of the "Journal of Applied Photographic Engineering". The article includes a graphic derivation of nonlinear tone reproduction curves encountered in practical electrophotographic print engines. The "Problems of Pictorial Xerography" article is hereby incorporated by reference exactly as set forth in full herein.
It is also known that generally the tone reproduction curves for electrophotographic print engines passes through the origin of the graph. It is well known that they may be shifted along the vertical axis (output density) in a manner which does not change their shape, but simply amounts to a translation along the output density variable by application of a bias voltage at the development station.
As noted herein above, the use of fine line or dot grid screens interposed in the optical path in an electrophotographic print engine is known in the art. Such screens have the effect of creating half-tone output images which significantly improve the perceived quality of a resultant pictorial image. However, by the inherent nature of the screening device, it reduces the exposure of the photoreceptor, thus increasing the density of the output image since less light is provided to the photoreceptor in areas corresponding to low density areas of the original image.
As explained above, the photoreceptive medium is blocked from illumination in those areas corresponding to the opaque areas of the screen. However, in blocking illumination of certain portions of the photoreceptive medium, the screen effectively reduces the intensity of the optical image. A reduction in the intensity of the illumination results in a general underexposure of the photoreceptive medium. In other words, the use of a screen reduces the optical images intensity and results in a lesser dissipation of the charges on the photoreceptive medium. Since less charge is dissipated, more toner will be attracted to the charged portions of the latent image resulting in more dense application of the toner, and in general, a darker, unacceptable reproduction of the original.
It is well known in the art that the relationship between exposure of the photoreceptive medium and the intensity of the light in the optical image may be expressed generally through the mathematical equation: Exposure=Intensity.times.Time. Since using a screen reduces the intensity of the light in the optical image, the effect of using a screen is to reduce the exposure of the photoreceptive medium in machines for which the exposure time is constant. For moving belt and drum machines, the exposure time remains unchanged unless the speed of the photoreceptor is varied, which is an undesirable approach. Reduction in the intensity of the light in the optical image results in an underexposed reproduction of the original.
One approach taught by the prior art to the problem of an underexposed copy when a diffusion screen is used is to increase the intensity of the imaging light source. Such a source may be a single higher wattage lamp or an additional lamp. However, this approach has several drawbacks. First, increasing the intensity of the imaging light source requires a lamp or other light source with a higher wattage rating, which is usually more expensive to buy, and to maintain. The higher wattage of the light source may require additional protective circuitry. Further, the power consumption of the imaging light source will increase. Increasing the intensity of the imaging light source also has the effect of heating the platen upon which the original is illuminated. This can raise platen temperature to an unacceptable level according to standards set by the Underwriters Laboratories. The approval of the Underwriters Laboratories is highly desirable for commercially marketed electrophotographic copying systems in the United States.
Another approach to the problem of a reduction in the level of exposure of the photoreceptive medium has been described in the patent to Goren, U.S. Pat. No. 4,007,981. Goren uses a secondary light source or lamp for additional non-image illumination. Goren teaches that this arrangement also produces acceptable half-toning of pictorial images. However, the use of additional lamps or other light sources has several drawbacks. An additional light source requires additional hardware and software support, thereby adding to the complexity of the copying system. The light source must be correctly positioned and its operation synchronized so that its light rays are in registration with the optical image. Further, the light source and its accompanying support system require at least routine maintenance. By adding an additional light source which operates simultaneously with the light source illuminating the original, the power consumption of the machine is increased significantly. All of these factors prohibit the production of a practical full color electrophotographic copying system which is of a size substantially equivalent to conventional table top monochromatic copiers and whose power consumption is controlled so that it may be operated from a conventional 120 volt branch circuit.
Accordingly, there is a need for a system which compensates for the reduction in the intensity of the optical image and the resultant reduction in the exposure level of the photoreceptive medium when a screen is positioned in the optical path, but which does not heat the platen to an unacceptable level. Further, there is a need for an exposure compensation system which can be utilized in a practical electrophotographic print engine which is the size of a conventional table top monochrome copier and whose power consumption is controlled so that it may be operated from a conventional 120 volt branch circuit. In the environment in which the present invention was made, the use of a secondary lamp such as taught by the aforementioned Goren patent, or increasing the light intensity at the imaging source, either by using a higher powered lamp or an additional lamp, was found to be unacceptable to meet the design goal of a copy machine which can be operated on a conventional 120 volt branch circuit.